PH I LI PS TECH N'ICAL REVIEWVOLUME 29, 1968, No. 7

The Philips helium liquefier

G. J. Haarhuis

The single-stage gas-refrigerating machine, described in this journal in 1954, provided asimple and efficient means of producing liquid air and nitrogen. With the advent of thetwo-stage machine, described in 1964, the liquefaction of hydrogen became a simple matter.Another step forward has now been taken with the construction of an efficient heliumliquefier based on a pair of two-stage gas-refrigerating machines. This liquefier, the prod-uct of close cooperation between a design team in the Industrial Equipment Division anda research team at the Philips Research Laboratories, is simple to operate and can processvery impure helium gas.

With the appearance in 1963 of a gas-refrigeratingmachine which gave high-efficiency refrigeration downto 20 "K, and could even reach 11 or 12 "K [11, thestage was set for the design of a helium liquefier basedon a gas-refrigerating machine. The inversion tempera-ture of the Joule- Thomson effect for helium lies between40 OK and 50 OK, so that it must be possible to achievethe last refrigeration step (down to 4.2 OK) by meansof an expansion (or throttling) process, just as in exist-ing helium liquefiers. The strongly increasing demandfor liquid helium for scientific and technical applica-tions, and the advantages associated with the use ofgas-refrigerating machines, prompted the decision toembark upon the development of a liquefier of thiskind. Partly through the invention at Philips ResearchLaboratories of a new type of heat exchanger with avery high thermal efficiency [21 and the expansionejector [31, and partly through the application of a newtype 0 f compressor equipped with pistons using a roll-ing diaphragm [41, a helium liquefier has emerged fromthis development work which possesses a number ofvery attractive features. The thermodynamic design isby Ir. G. Prast of the Philips Research Laboratories.

The new liquefier system, which is very compact and

Ir. G. J. Haarhuis is with the Philips Industrial Equipment Division(PIT), Eindhoven.

(once started up) automatic in operation, delivers about10 litres of liquid helium per hour, and if the heliumcontains 2% of air it can still operate continuously for100 hours without requiring cleaning. Even if the per-centage of air is much higher the operation of the lique-fier remains unimpaired, and an air content of as highas 10% is in fact permissible for a limited time. If thehelium used contains less than 2% of air, the period ofcontinuous operation is well over 100 hours. Once theliquefaction process has started, all the operator has todo is to ensure that the liquefier is regularly suppliedwith helium gas, and to replace full Dewar vesselsby empty ones. The efficiency of the system is high: theproduction of one litre of liquid helium only uses up2.8 kWh of electrical energy, including the energyrequired for removing the 2% of air. Unlike other lique-fiers, this one does not require a supply of liquid nitro-gen for precooling or purification of the gas supply.

[1] G. Prast, A gas-refrigerating machine for temperatures downto 20 "K and lower, Philips tech. Rev. 26, 1-11, 1965.

[2] G. Vonk, Acompact heat exchanger of high thermal efficiency,Philips tech. Rev. 29, 158-162, 1968 (No. 5).

[3] J. A. Rietdijk, The expansion ejector, a new cryogenic device,Philips tech. Rev. 28, 243-245, 1967 (No. 8).

[4] For the roIling diaphragm, see: J. A. Rietdijk, H. C. J. vanBeukering, H. H. M. van der Aa and R. J. Meijer, A positiverod or piston seal for large pressure differences, Philips tech.Rev. 26, 287-296, 1965.

198 PHILTPS TECHNICAL REVIEW VOLUME 29

Construction and operation

Fig.il shows a simplified diagram of the liquefier. Acompressor compresses the helium gas at room tem-:perature from 2.5 bar to 20 bar (gas flow rate 2.5 gis;. 1 bar is about 1 atm). The compressed gas passes thefive gauze-type heat exchangers HI to H5 and the pre-coolers RI to R4 of a pair of two-stage gas-refrigeratingmachines. In the last heat exchanger a temperature of7 "K is reached. The helium leaving the system in aliquid state is replaced by helium gas supplied throughthe tube SI at a pressure of 20 bar. This gas flow firstpasses through all five heat exchangers before joiningthe main flow (at' Con). The cold gas expands in theexpansion ejector EE to 2.5 bar and is cooled downfurther by the Joule-Thomson effect. A large propor-tion of this gas flows through the five heat exchangersH5 to HI back to the compressor. ,On its way it givesup nearly all its cold to the two other flows passingthrough the heat exchangers. The part of the cold gasthat does not flow back undergoes a second expansionin the expansion valve JK, this time to a pressure of1 bar, and a certain fraction liquefies during this ex-pansion. Gas and liquid then flow to vessel C, and thegas then returns from C to the main flow via the ex-pansion ejector. The whole process described here iscontinuous.During the period immediately after the system is

switched on, no helium is supplied through SI. Gascirculates then only in the middle and left-hand tubesshown in HI to H5 in fig. 1. The system then graduallycools down. When the temperature between H2 and H3has dropped to 80 "K, this starting period is consideredto be over and the gas supply starts through SI. Thedecrease of pressure in the system as a result of thecooling during the starting period is compensated bythe supply of pure helium through S2.In the actual system the vessel C is a Dewar flask in

which the liquid helium produced is collected andstored. The components inside the chain-dotted rectan-

Fig. 1. Simplified diagram of the Philips helium liquefier. Compcompressor. RI to R4 four precooling stages obtained from twotwo-stage gas-refrigerating machines. HI to H5 gauze-type heatexchangers. EE expansion ejector. JK expansion valve. Ha heatexchanger. C vessel for collecting liquid helium. Fresh gas isregularly supplied to the system through tube SI; it passesthrough the heat exchangers HI to H5 and then joins the mainstream at COli. Impurities in the gas supply are eliminated at theplaces denoted by the relevant chemical symbol. The part insidethe chain-dotted line is contained in a special Dewar vessel.

gle are contained in another Dewar flask, the cryo-stat. This is in communication with C by means of aspecially constructed connecting system with coaxialfeed tubes .

Special features

The chief feature that distinguishes the diagram offig. 1 from that of any other liquefier is that it includesthe expansion ejector EE (fig. 2). This works both asan expansion valve and as a pump of the water-jet type:

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1968, No. 7 HELIUM LIQUEFIER 199

it draws the helium gas at 1 bar from the vessel C andraises.it toa pressure of 2.5 bar. This is at the sametime the suction pressure of the compressor, which cantherefore be very much smaller than if it had to drawthe gas from C. This is important for efficiencyand alsosaves space.

Fig. 2. Cross-section of the expansion ejector. The helium gasflowing from above (20 bar) expands to 2.5 bar, giving coolingthrough the Joule-Thomson effect. At the same time, through theopening on the left, helium gas of 1 bar is drawn in from thecollector vessel (C in fig. I) and again compressed to 2.5 bar.

The gas flowing from the ejector EE to the expansionvalve JK is cooled in the heat exchanger H6 by the gasflowing back from the vessel C to the ejector. It there-fore arrives at JK with a temperature of only 5.2 "K,and because of this Iow temperature no less than 60 to70% of the gas is liquefied during the expansion at JK.The pair of two-stage gas-refrigerating machines with

which the liquefier is equipped work at different tem-peratures and thus drive four precooling stages. Twomachines have to be used since the output yield withone machine ofthe conventional type (A20) is less thanthe amount considered desirable. The use of more thanone machine has two other important advantages,however. In the first place, in cooling a flow of gas orliquid the highest efficiency is.in t~eory obtained whencold is supplied to the medium at each temperature in

the cooling process. This ideal situation is much betterapproximated when cold is supplied at four differenttemperatures - here 100, 65, 30 and 15 "K - thanwhen it is supplied at only one or two. In the secondplace, when cold is supplied at four temperatures it ispossible to remove the impurities from the helium gas

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(The dashed parts of the curves correspond to situations inwhich the head is less cold than the other stage; such situationsdo not of course arise when the machine is in normal use.)

(mainly air) in a very effective way by adsorption. Thiswill be discussed separately in the next section..A feature of the two-stage machine which we have

found very useful is that the cold production of thefirst stage - i.e. the one with the higher tempera-ture - has hardlyany effect on that of the second stage(fig. 3). The 'system can thus be arranged so that a

200 PHILIPS TECHNICAL REVIEW VOLUME 29

large part of the cold is delivered at the higher tempera-ture, which improves the 'efficiency.

We have already mentioned the compressor withroIling diaphragms, specially designed for this lique-fier. This compressor, one of a type designed byIr. H. J. Verbeek of the Philips Industrial EquipmentDivision, will be the subject of a forthcoming article inthis journal. The roIling diaphragms form a hermeticseal between the spaces containing the gas for compres-sion and the spaces containing oil. This is of particularimportance: without an oil-free compressor a systemlike that described here would 110t be able to workcontinuously for very long.

Purification of the gas supply

At the temperature of liquid helium' all other sub-stances are solids. Impurities in the gas supplied musttherefore be removed before this temperature is reached,otherwise they can cause stoppages.' In the Philipshelium liquefier this is done by adsorption, as we notedearlier. With one exception, the adsorbers are all loc-ated in the cryostat, each being placed at the coldestpossible place consistent with the requirement that theimpurity in question is to remain in the gas phase (cf.fig. I). The new helium liquefier differs in this respectfrom all existing ones, which require pre-purificationof the helium supply to reduce the impurity content toabout 0.01 % by volume. This pre-purification requiresthe use of liquid nitrogen, which is not required in thePhilips liquefier.

As can be seen from fig. 1, the adsorber that trapswater vapour works at room temperature. Carbondioxide gas is trapped by an adsorber situated betweenthe first and second heat exchangers, where the pre-vailing temperature is about 125 OK. Both adsorberscontain "molecular sieves", and if the helium gasentering through SI contains 2% of air and is saturatedwith water vapour, they must be regenerated once every100 hours. . . .

Oxygen, nitrogen and argon are removed between thesecond heat exchanger and the third (temperature about70 OK). This is done by means of two adsorbers con-sisting of activated charcoal which operate alternatelyfor periods of 40 minutes. While one is working theother is being regenerated, which is done by raising thetemperature to about 145 OK and pumping off the de-sorbed gàs. The switch-over is automatic. If the gassupply contains more than 2 % of air, part of it is lique-fied in the second heat exchanger. This liquid is col-lected in a Dewar vessel and from time to time auto-matically removed from the system (see below).

Finally, between the fourth heat exchangers andthe fifth there is another small charcoal adsorber whichtraps hydrogen and neon; the quantity of these gases

contained in the gas supply is usually very small. Thisadsorber can also work for 100 houts continuously with2% of air in the helium supply. -

All these adsorbers are located in the supply line(SI-Con) so that the gas circulatingin the startingperioddoes not pass through them. For purifying this gas aseparate adsorber is included between H2 and H3. Asthe temperature of the system falls, this adsorber trapsan increasingly larger fraction of any impurities, andeventually the circulating helium is practically pure.

Technical details

The construction of the new liquefier is illustratedschematically in jig. 4. The photograph (fig. 5) givesa general view of the system. The cryostat, which con-tains the five gauze-type heat exchangers, the ejector,the expansion valve and the devices for purifying thegas (except the adsorber for water vapour) are con-tained inside an evacuated Dewar vessel (1-0.1 torr) ofspecial design. This vessel is divided into three compart-ments by two refrigerated radiation shields RSI andRS2, coated with silver on both sides; the shields extendinto the high-vacuum space between the two walls ofthe Dewar vessel. Each shield is cooled by a freezer ofone of the refrigerating machines; RSI has a tempera-ture of about 100 OK, and RS2 has a temperature of30 OK. The use of these shields considerably improvesthe heat insulation of the Dewar vessel. The part of theshield situated between the walls of the Dewar vesseland the part in the open space are made separate as theequipment has to be demountable. The rim ofthe shieldis increased in area by means of a special strip and thegap between the two parts is kept small (about 0.25mm), thus ensuring sufficient heat transfer from con-duction in the gas.

The expansion valve JK is automatically controlled.At the top of the cryostat there is a double-bellowssystem, not shown in fig. 4, which is connected with theline containing the helium that has just passed the .expansion valve: The position of the bellows is deter-mined by the difference between the pressure in the line(normally just above 1 bar) and the pressure of theoutside air. When the pressure in the line increases, thebellows system closes the expansion valve tighter; whenthe pressure decreases, the valve opens wider. Thissimple control method proves eminently satisfactory inpractice.

At the centre of fig. 4, next to the air adsorbers, acomponent BI can be seen which we have notyetmen-tioned. This starts to operate when the helium gas sup-plied to the liquefier is so strongly contaminated withair that air begins to condense in the heat exchanger-He.This liquid air is collected in BI, and automaticallyblown out again when the liquid reaches a certainlevel.

1968, No. 7 HELlUM LIQUEFIER 201

Fig. 4. Complete constructional diagram ofPhilips helium liquefier. Dew walls of Dewar vessel (cryostat). RSi and RS2 refrigeratedradiation shields at 100 OK and 30 OK respectively. D delivery tube, containing one channel for removing the liquefied helium and onefor the returning gas. B buffer vessel. Pi and P2 vacuum pumps. The various adsorbers for removing impurities are again denotedby the chemical symbol of the gas which they adsorb. BI device for collecting and automatically blowing off liquid air; thiscomes into operation only when the impurity content of the helium gas is very high (see also fig. 6). The other letters have the samesignificanee as in fig. I.

202 PHILlPS TECHNICAL REVIEW VOLUME 29

Fig. 5. The Philips helium liquefier. One Philips two-stage gas-refrigerating machine is to beseen on the left in the background and another can be seen at the far right. A Dewar vesselin which the liquid helium is collected and removed can be seen in the foreground. The com-pressor is behind the right-hand refrigerating machine. The cryostat is at the centre of thepicture; the cabinet attached to it on the right contains meters for registering temperaturesand pressures.

Details of this automatic system are given in fig. 6,together with the air adsorbers and the correspondingvalves, feeder tubes, etc. The vessel BI contains a hollowpin, which is open at the bottom and is fitted inside with

a reed relay. Floating on the liquid air around the pin isa ball F with a permanent magnet inside it. If the liquidlevel rises there comes a moment at which the magnet islevel with the relay reeds, which are ferromagnetic. The

1968, No. 7 HELIUM LlQUEFTER

reeds then spring together, the valve V3 opens andremains open until F has gone down far enough for therelay to open again. Things are so arranged that nohelium gas can be blown off. The air adsorbers Al andA2 are brought into the circuit alternately by means ofthe pneumatically operated double-acting valves VI andV2. When an adsorber is being cleaned, the desorbedgas escapes through the tube shown in the drawing atthe top, which leads to a vacuum pump.

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A 4 is the adsorber which purifies the gas circulatingin the system during the period immediately after thesystem has been switched on.

The last part of the new liquefier we shall discuss isthe delivery tube, i.e. the tube (D in fig.4) throughwhich the liquid helium leaves the cryostat and flowsto the Dewar (fig. 7). In the sleeve 1, which can be seenin the foreground offig. 5, there are two coaxial tubes 2and 3, which are surrounded in a part of the delivery

Fig. 6. Apparatus for removing nitrogen, oxygen and argon from the helium gas supply.H2 and H3 gauze heat exchangers (see fig. I). Ai and A2 groups of two adsorbers connectedin series, one of which is in operation while the other is being regenerated. The adsorbed gasescapes through the tubes shown at the top, which lead to a vacuum pump. hand V2 aredouble-acting pneumatically operated valves for switching over from one group of adsorbersto the other. A3 adsorber for use during the short transitional period after switching over.H7 heat exchanger. BI collector vessel for the air condensed in H2, with a device (F, T) forautomatically opening the valve V3.

Adsorber A3 is also an air adsorber, which comes intooperation immediately after Al has been switched overto A2 or vice versa. The adsorber just switched in isthen still relatively warm, and during the first fewseconds does not trap anywhere near all the air thatpasses. This air is adsorbed by A3, which is alwayssufficiently cold because the gas flow from AI,2

(0.35 gis) first passes the heat exchanger H7 throughwhich the main flow (2.5 gis) also passes. The timeduring which the liquefier can operate continuously ismainly determined by the capacity of A3.

tube by a refrigerated radiation-shield 4, silvered on theoutside. In the space between 3 and 1 there is a highvacuum. Gas and liquid flow through the inner coaxialtube to the collector vessel (in fig. 1 the part of the linebetween IK and C), and gas flows back to the cryostat(line between C and EE in fig. 1) through the outer tube(shaded). The radiation shield 4 is connected with oneof the radiation shields of the cryostat, i.e. the shieldat about 100 "K. The end of the delivery tube project-ing into the collector vessel (on the right in the figure)contains valves 6 and 7, which can be pneumatically

203

204 PHILlPS TECHNICAL REVIEW VOLUME 29

Fig. 7. a) Delivery tube;the end at the right reaches into thevessel where the liquid product is collected, the other end intothe cryostat. 1 outside wall. 2 central tube for the removal ofliquid and gas.3tubethrough which gas flows back to thecryostat,coaxial with 2. 4 radiation shield (lOOOK) coaxialwith 2 and 3.5 bellows system for closing the tube, working pressure 2.5 bar;the pin 6 closes tube 2, .the ring 7 closes the holes in block 8through which the gas can enter tube 3. 9 compression spring.b) Connection of the tube to the cryostat. As long as the tube isnot in the lowest possible position, a ball valve, with balll0 andspring 11, prevents the escape of helium gas and liquefied heliumcoming from the expansion valve (IK in fig. I and fig. 6). Whenthe tube enters the cryostat to the fullest possible extent, it pressesthe ball 10 downwards and the helium then has access throughholes 12 to the central tube 2. The gas returning to the cryostatcan flow to the expansion ejector (EE in fig. I and fig. 6) viahole 13 and tube 14.15 rubber bellows; this enables the tube tobe raised for changing the container, without resulting in an openconnection between the ejector and the outside air.

operated by means of the bellows system 5. The otherend of the tube reaching into the cryostat can easily betaken out, which automatically closes the line whichsupplies the liquid and gas to the central tube. A specialmethod of manufacture is used for making the complexassembly of coaxial walls 1, 2, 3 and 4: the structureis fabricated in rectilinear form, all the spaces arefilled with water, and it is then cooled in a suitableway to the temperature of liquid nitrogen and bentto shape. -

8

79

Summary. The article describes an easily operated, highly auto-matic helium liquefier which makes use of Joule-Thomson effectcooling and has a pair of two-stage gas-refrigerating machines forprecooling. The installation produces about 10 litres of liquidhelium an hour at a high efficiency (power consumption 2.8 kWhper litre). The helium gas does not have to be carefully purifiedbeforehand. If the gas contains 2% of air, the system can workfor 100 hours continuously; a much higher air content can betolerated for short periods of operation. Factors contributing tothe high efficiency are the availability of four precooling tem-

peratures (approx. 100, 65, 30 and 15 OK) and the use of specialgauze-type heat exchangers and an expansion ejector. The ex-pansion ejector permits the use of a relatively small compressor(suction pressure 2.5 instead of I bar, and compression 20 bar).This compressor is equipped with rolling diaphragms. All impuri-ties are removed by adsorption, each at the most appropriatetemperature. The connection between the cryostat and the vesselin which the liquid helium is collected is formed by a speciallydesigned delivery tube, containing coaxial feeders for the liquidproduct and the returning gas.

."